In this paper, the design of nonlinear softening springs using compliant mechanisms is investigated. The use of compliant structures is of great interest, because of the resulting absence of backlash and friction. We demonstrate that the existence of parallel singularities is a necessary condition for the architecture of a compliant softening spring. From this result, two original arrangements of softening springs are derived, with the introduction of traction and torsion softening springs. A synthesis is performed and the traction spring is numerically and experimentally assessed. As nonlinearity can also be obtained from material properties, the interest of using additive manufacturing with multi-material capability is investigated. Rubber-like materials exhibit a hyper-elastic behavior. Their integration in the proposed compliant architecture is shown to be of interest to customize the geometry of a softening spring according to the designer requirements.